Multi-phase polyolefin compositions—often consisting of a “plastic” matrix phase and a “rubber” dispersed phase—are used in many applications that require a material that is lightweight, tough, stiff, and easily processed. Particularly successful in this respect are compositions based on a high-modulus polypropylene and a low-modulus polyolefin modifier, which are typically referred to as thermoplastic polyolefins, or TPOs. The modifier component has elastomeric characteristics and typically provides impact resistance, whereas the polypropylene component typically provides overall stiffness.
Commercially, the most common example of a TPO is a polypropylene impact copolymer (ICP). In an ICP, the matrix phase is essentially a propylene homopolymer (hPP) or random copolymer (RCP), and the dispersed phase is typically an ethylene or propylene copolymer with a relatively high comonomer content, traditionally an ethylene-propylene rubber (EPR). A third phase containing mostly comonomer, such as an ethylene homopolymer or copolymer (PE), as well as additives such as fillers, may also be present.
Low-crystallinity ethylene-alpha olefin copolymers have also been used instead of (or in addition to) EPR as the polyolefin modifier (or dispersed phase) in TPOs. The most common such ethylene-alpha olefin copolymers are so-called plastomers, which are often ethylene-butene, ethylene-hexene, or ethylene-octene copolymers with densities of 0.90 g/cm3 or less.
A major market for TPOs is in the manufacture of automotive parts, especially exterior parts like bumper fascia and body side-molding, and interior parts like instrument panels and side pillars. These parts, which have demanding stiffness and toughness (and, in some cases, uniform surface appearance) requirements, are generally made using an injection molding process. To increase efficiency and reduce costs, manufacturers have sought to decrease melt viscosity, decrease molding times, and reduce wall thickness in the molds, primarily by turning to high melt flow rate (MFR) polypropylenes (MFR greater than about 20, 25, or even 30 dg/min). However, these high MFR polypropylenes tend to be low in molecular weight, and therefore difficult to toughen, resulting in low impact strength especially at sub-ambient temperatures. To achieve a satisfactory balance of stiffness, toughness, and processibility, one option is to combine a moderate MFR polypropylene, a high content of polyolefin modifier (typically EPR and/or plastomer), and a reinforcing filler. Unfortunately, this approach has limitations in terms of the maximum MFR that can be achieved while still meeting the stiffness and toughness requirements. In addition, it can lead to poor surface appearance, in terms of the appearance of flow marks (or “tiger stripes”).
What is needed is a way to improve the MFR characteristics of a TPO without sacrificing its level of performance in terms of mechanical properties, including impact strength and/or stiffness, that are demanded for applications like automotive parts. One way to increase MFR is to add a low molecular weight compound, such as an even higher MFR polypropylene (hPP, RCP, or ICP). However, this approach compromises the balance of mechanical properties of the final blend by also lowering its sub-ambient impact strength. A fundamental problem is that typical TPO's based on polypropylene are brittle at even moderately low temperatures due to its relatively high glass transition temperature (˜0° C.). Therefore, there is a need for a low molecular weight additive that also lowers the glass transition temperature of the polypropylene component of a TPO. The plasticizers described herein accomplish this objective.
It would be particularly desirable to use a simple compound such as a conventional mineral oil as the low molecular weight additive for this purpose. After all, such compounds are routinely used as process oils or extender oils in polyolefin elastomers. However, it has been taught that conventional mineral oils, even paraffinic mineral oils, impair the properties of polyolefins, in particular semi-crystalline polyolefins (see WO 01/18109 A1 and Chemical Additives for the Plastics Industry, Radian Corp., 1987, p. 107-116). Indeed, such compounds are often detrimental to semicrystalline polypropylene, in that they migrate to the surface causing parts to become oily (except at very low concentrations), or they degrade mechanical properties because they fail to depress the glass transition temperature effectively. The plasticizers described herein overcome these limitations.
WO 04/014998 discloses blends of polyolefins with non-functionalized plasticizers. In particular, Tables 8, 11, and 21a to 22f describe blends of certain impact copolymers with certain liquids and/or plasticizers, and Tables 23a to 23f describe blends of a certain thermoplastic polyolefin composition with certain liquids and/or plasticizers. These blends however are unsuitable for automotive TPO applications because they do not have the appropriate balance of stiffness, toughness, and flow properties.
Plasticized polyolefin compositions and their applications are also described in WO 04/014997 and US 2004/260001. Additional references of interest include: U.S. Pat. Nos. 4,132,698, 4,536,537, 4,774,277, JP 09-208761, WO 98/44041, WO 03/48252, and US 2004/034148.
TPOs, including compositions that comprise polypropylene and or filler, are described in POLYPROPYLENE HANDBOOK, 2ND ED., N. Pasquini, Ed. (Hanser, 2005), p. 314-330; POLYMER BLENDS, D. R. Paul and C. B. Bucknall, Eds. (Wiley-Interscience, 2000), Vol. 2; U.S. Pat. Nos. 5,681,897; 6,245,856; and 6,399,707. However, the addition of both a filler and a non-functionalized plasticizer to a polypropylene-based TPO to give an improved balance of properties, as described herein, has not been previously disclosed.